Such a ball and socket joint is described in the German patent application DE 100 09 054, in which the ball and socket joint housing made of an electrically conductive material and the ball pivot are connected via electric lines with an evaluating unit, by means of which excessive wear of the bearing shell, which is characterized by a direct contact between the joint ball and the ball and socket joint housing, can be detected.
DE 195 46 084 C1 discloses a ball and socket joint, e.g., for the suspension of a front wheel of a motor vehicle, in which a ball pivot with a ball head is arranged in a bearing housing through the intermediary of a bearing shell. The ball pivot and the bearing housing are made of a metallic material, whereas the bearing shell is made of a plastic, which does not conduct the electric current. A voltage is applied between the ball pivot and the bearing housing for measuring the wear, and the flowing current is measured.
Furthermore, a ball and socket joint, e.g., for the steering means of a motor vehicle, is known from JP 56006911 A, in which a ball pivot with a joint ball is arranged in a joint housing through the intermediary of a bearing shell. The joint housing is made of a metallic material, whereas the bearing shell is made of an insulating plastic. An electrode made of copper is embedded in the bearing shell, and this electrode does not at first extends to the inner surface of the bearing shell and is not in electric contact with the joint housing, which is connected with a power source via an electric line through the intermediary of a signaling device. When the bearing shell is worn, the electrode comes into direct contact with the joint ball of the ball pivot connected to the ground, whereupon the signaling device is activated.
Greases, which are mixed, for example, with graphite or molybdenum and are thus electrically conductive, are usually used in ball and socket joints for motor vehicles. Furthermore, contaminants, including water, which are likewise conductive, can penetrate into the ball and socket joint. It is thus possible that these substances enter the area between the electrode and the joint ball of, e.g., a ball and socket joint according to JP 56006911 A and form an electric contact resistance there, which has a relatively low resistance because of the short distance between the electrode and the joint ball. However, when an electric current can flow between the electrode and the joint ball, even though the bearing shell does not yet permit a direct contact between the electrode and the joint ball, i.e., the state of wear has not yet developed, this may lead to measuring errors during the determination of the state of wear.
On the other hand, these contaminants may lead to corrosion of the electrode over the lifetime of the above-mentioned ball and socket joint, so that no electric contact or only a weak electric contact will develop in case of wear, i.e., in case of a direct mechanical contact between the electrode and the joint ball. This may also lead to measuring errors during the determination of the state of wear.